1. Field of the Invention
The invention relates generally to a process for the alkylation of aromatic compounds, e.g., toluene methylation, with a zeolite catalyst, e.g., an aluminosilicate zeolite which has been modified to be a shape selective catalyst, said process having reactors in series and more specifically relates to a process to produce para-xylene (p-xylene) by toluene methylation with a shape selective zeolite catalyst in a series of reactors with interstage separation of C8+ aromatics.
2. Description of the Prior Art
Toluene methylation (TM) is a catalytic reaction of toluene with methanol to produce xylenes as shown below:
Toluene methylation is an equimolar reaction, i.e., one mole of toluene reacts with one mole of methanol.
Zeolites which are crystalline solids made up of aluminum-substituted SiO4 tetrahedral units joined together to form different ring and cage structures into a crystalline framework can be used as catalyst for toluene methylation. The physical structure of zeolite is very porous with a large internal and external surface area. Zeolites can be shape-selective catalysts due to steric and electronic effects. Shape selective properties can be obtained by modifying the zeolite, e.g., narrowing zeolite pore opening size, inactivation of the external surface of the zeolite or altering zeolite acidity. Deposition of certain compounds or elements on the zeolite can make it more shape selective, e.g., compounds containing iron, zinc, phosphorus, rare earth metal oxides, etc.
In the synthesis of p-xylene by methylation of toluene, the conversion of toluene and the selectivity of p-xylene, i.e., concentration of p-xylene in the xylene isomers are of commercial importance. Para-xylene (PX) is of particular value as a large volume chemical intermediate in a number of applications being useful in the manufacture of terephthalates which are intermediates for the manufacture of PET. It would be advantageous for a process to produce p-xylene at concentrations of at least 85% or at least 90%.
In addition to the methylation of toluene, many competing side reactions can occur. Methanol may react with itself to form olefins. Toluene can be over-alkylated to form C9+ aromatics.
U.S. Pat. No. 4,761,513 discloses a multistage process for alkylation of aromatic hydrocarbons in which the temperatures are controlled by adding the alkylating reactant into each reactor proportionately in both liquid and vapor phase to provide a quench to control temperatures in the reactors. Alkylate product is recovered from the last reaction zone.
U.S. Pat. No. 6,642,426 discloses a process for alkylation of aromatics in a fluidized bed reactor in which a portion of the alkylating reagent is introduced with or near the aromatic reactant and a portion is injected downstream from where the aromatic reactant is introduced, such as into the fluidized bed along the flow axis of the reactor or into a region between two separate discrete fluidized beds.
U.S. Pat. No. 4,377,718 discloses a multistage process for producing p-xylene in a plurality of separate series-connected fixed catalyst layers with toluene being fed with hydrogen into only the first stage and passed successively through each subsequent fixed catalyst layer and the methylating agent being fed into each fixed catalyst layer.
U.S. Pat. No. 7,321,072 discloses a process for producing p-xylene by selective methylation of toluene in a flow reactor at a contact time between a reactant mixture of toluene, methanol and added water and a zeolite catalyst of less than 1 second at a temperature of from 250 to 500° C. The process may be carried out in fixed, moving or fluid catalyst beds, either individually or connected to form multiple bed catalytic reactors. Optionally, in a multiple bed arrangement, additional methanol can be introduced into the reaction mixture between beds to improve conversion.
A process which would increase p-xylene concentration in the mixed-xylene product stream so that separation of p-xylene from o-xylene and p-xylene can be more easily accomplished would be advantageous.